On Sat, 03 Sep 2011 11:31:51 -0700, Richard Clark
wrote:
On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:
The two 47K resitors going to 0.01uf bypass caps make
an effective ground to any fast risetime voltage spike at the antenna.
I'm sure you are perfectly aware of the single point of failure in
that generality.
I am? Ummm... well, I guess so.
Few Caps exhibit 0.01uF (when so marked) to transients (where it is
presumed they will exhibit 1/2*pi*f*c reactance to the risetime).
True. They all have some internal resistance to overcome. However,
that's negligible resistance when compared to that of a static blast.
Static electricity has lots of potential (volts), but is only able to
deliver small amounts of current. That's why we don't get
electrocutes by the potential (voltage) difference between our head
and our feet. Dividing the large voltage, by the tiny current,
results in a fairly substantial source resistance. I'm too lazy to
look it some real numbers, but I'm sure it's in mega ohms. The 47K
resistance, and whatever ESR the 0.01uF contributes, has little effect
on the energy delivered to the shottky diode.
Incidentally, if the source resistance of the static blast was much
less, then the diode would not simply be fried. It would probably
explode.
When we (silverbacks) got into this game, (the preferable) mica caps
were available, snipped out of the nearest sacrificial TV or radio.
Trying to read those several styles of color coding was the biggest
hurdle, but I had plenty in my junk-box.
Dumpster diving in Henry Radio's trash can in West Smog Angeles was
one of my favorite after skool exercises. Salvaging old TV chassis
and dead tubes were the grand prizes. Silver mica caps came a close
second.
Ceramic is ubiquitous, now, and far from choice in these matters,
unless you do deep research (maybe).
Ceramic is cheap. I was a big fan of porcelain caps from AVX in big
power amps. If you wanna handle current, there's nothing better.
Silver mica would get hot, ceramic would explode, and everything else
was either too big or too expensive.
Incidentally, I don't think they make 0.01uF silver mica caps. The
biggest I played with were in antenna tuners at 4700pF (or should I
say uuF for nostalgia purposes).
You got any favorites that respond to this?
I don't understand the question. Favorite what?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558